Daniel Stashower was flipping through comic books in a friend’s tree house when he learned about a mysterious, unsuccessful plot to kill Abraham Lincoln. The tale of Lincoln’s narrow escape from an attempted murder in Baltimore before his 1861 inauguration appeared in a true-crime comic, and Mr. Stashower, then 13, was hooked.

His obsession never faded. Last night, Mr. Stashower took home the Edgar Award for Best Fact Crime at the Mystery Writers of America awards for his 2013 book “The Hour of Peril: The Secret Plot to Murder Lincoln Before the Civil War.” His account of the so-called Baltimore Plot details how detective Allan Pinkerton teamed up with the country’s first female private eye, Kate Warne to unravel a plot to kill Lincoln as he traveled by train from Springfield, Ill., to his inauguration in Washington D.C. Bestselling crime writer Harlan Coben called it “history that reads like a race-against-the-clock thriller.”

Mr. Stashower has won two previous Edgar awards, including one for his biography of Sherlock Holmes creator Arthur Conan Doyle, but his award last night was historic for his publisher, Minotaur, which took home its first award in the true-crime category. (“The Hour of Peril” was the first nonfiction book to come out of Minotaur, a crime imprint that launched in 1999 and has won nine Edgar awards for fiction.)

Other winners last night included William Kent Krueger, who won in the best novel category for “Ordinary Grace,” about a 13-year-old boy in Minnesota who struggles to cope with multiple tragedies in the summer of 1961; Jason Matthews, whose modern day Russian spy novel “Red Sparrow” won for best first novel by an American author; Alex Marwood, who won for best paperback original for her twisted murder story “The Wicked Girls,” and Annabel Pitcher, whose novel “Ketchup Clouds,” about a 15-year-old girl who becomes pen pals with an inmate on death row, won in the young-adult category. Read the full list of winners here.

Minotaur Books/Associated Press

Mr. Stashower talked with Speakeasy by phone as he was traveling on a train to the Malice Domestic mystery convention in Bethesda, Md., where he lives. Below is an edited transcript of the conversation.

So you first heard about this historic plot to kill Lincoln when you were 13. What drew you back to the story so many years later?

A lot of roads led there. I’ve always been interested in true crime and history. I worked at Time Life books on a Civil War series, and I was exposed to some of the players in the drama. There were a lot of different threads that came together. What really intrigued me was a story about a woman named Kate Warne, who worked for Allan Pinkerton. The notion that there was a female detective doing important dangerous work alongside men in the field in the 1850s was fascinating to me. She was a central figure in the Baltimore Plot. She was at his side that night.

This is a story that’s disputed among historians and even some of the key players deny that it happened. What made you believe in the plot?

Historians are going to debate the details of what happened in Baltimore until the end of time, but the existence of a plot in Baltimore is absolutely beyond dispute. Pinkerton himself, in a memoir, wrote about the episode, and was very clear about what he expected to happen. Lincoln was traveling from Springfield to Washington, D.C., and there was no direct line. He had to make the trip on 18 different railroad lines, and these lines didn’t join up, so Lincoln had to get up and travel through the town. There would be a parade. He wanted to use this journey to washington to give people a chance to see his face, but it also created a security nightmare. I feel it is absolutely ludicrous to suggest that there was no danger.

With so many conflicting accounts, how did you figure out what parts of the story were true?

There has been a great deal written on all sides of the story over the years. What is fascinating to me is the way in which the telling of he story became a feature of it almost from the beginning. From the moment it occurred, the three men involved had reasons to conceal it. For Lincoln, it was a public relations disaster. Lincoln felt he would be ridiculed if he fell in with Pinkerton’s clan, and he was. He was eager to close the file and move on. Pinkerton was cagey with his details because that’s how he did business. Another player, [Lincoln’s friend and body guard] Ward Hill Lamon, he and Pinkerton just rubbed each other the wrong way. He became determined that Pinkerton should be deprived of any credit.

A key figure in this story is Kate Warne, who is described as America’s first female private eye. Had her role in the event been minimized historically, and did you hope to bring greater recognition to the part she may have played in saving Lincoln?

I certainly hope to. It’s absolutely one of the most interesting things about the story for me. It would be decades before women would have the right to vote, and here’s this woman working for Allan Pinkerton. She knocked on his door one day and said I’m looking for work. He said something like, “It’s not the custom to employ women as detectives. How would you be of service?” She was ready for this and she said, I’m paraphrasing, “A woman may go and ferret out secrets in ways that are impossible for male detectives.”

A number of reviewers, including crime novelists like Harlan Coben, praised this book by saying it read like a suspense novel or thriller. How did you give it the flavor of a novel without bending the facts? Did you fill in narrative detail when it wasn’t available?

That’s a very slippery slope and you cannot make this stuff up. While I struggled very hard to give it narrative drive and structure, the challenge was to take it from sources that you can point to, including Pinkerton himself, who wrote and commissioned dramatic retellings of his episodes. It’s fair to treat those accounts with some suspicion, but it’s fair to use them, because they’re written by people who were there.

The Weinsteins optioned the book for TV, for a mini series, is that happening?

I don’t know. I certainly have my finger’s crossed.

What are some of your favorite true crime books?

[Truman Capote’s] “In Cold Blood” is still one of my favorite books of all time. It’s the most incredible combination of writing and storytelling, and it was a book that just changed my world. Of course I’m a big fan of Erik Larson (“The Devil in the White City”) and James Swanson (“Manhunt”). It seems as if we’re in a real renaissance for true crime writing.

What are you reading now?

My 14-year-old son is reading “To Kill A Mockingbird.” Of course I’ve read that book before, but I’m reading it again so that I can talk about it with him, and it’s even better than I remember.